What would you do if lets say you get a great idea and you are currently working on that great idea?

and you find that there are people who are also may be thinking or working on that great idea (maybe not in the same line or context as yours (you dont know that too)..but should you then work on that idea or not? To specify lets say i am writing a book, and i come to know that there are other people also working on my chosen topic in different corners of the globe...so should i give up the idea or think of something else or what should I do? I dont know if my book will be successful or not..I know nothing....so what should I do? Please help